Thermosensitive diblock copolymer of poly(N-isopropylacrylamide) and poly(ethylene glycol) in water: Polymer preparation and solution behavior

This investigation focused on the self-assembly of poly(N- isopropylacrylamide)-eZoc&-poly-(ethylene glycol) (PNIPA-block-PEG) in water. A quasi-living radical polymerization technique including a Ce(IV) ion redox system enabled us to prepare block copolymers with relatively narrow molecular weight distributions. We distinguish five regions in the phase diagram: a transparent sol, opaque sol, transparent gel, opaque gel, and syneresis. By examining the extent of changes in the spectroscopic properties of a fluorescence probe, pyrene, as a function of block polymer concentration and/or temperature, we determined the critical association concentration as well as the partition coefficient K_v for pyrene. The spectroscopic properties indicate that the hydrophobicity around the probe starts to increase far below the demixing line of the PNIPA-block-PEG, a remarkable finding which suggests that even in the temperature region below the LCST temperature of a PNIPA block (∼32°C), this block copolymer provides more space for a preferential transfer of pyrene molecules than a bulk water medium at a higher temperature. This result may be attributed to the action of water, which starts to behave as a selective solvent for PEG blocks; the PEG chains are more swollen with water than are the PNIPA chains. Dynamic light scattering measurements also indicate that contraction of the PNIPA block starts to occur around 18°C, which is consistent with results obtained by fluorescence measurements. By employing small-angle neutron scattering, it is also confirmed that microphase separation occurs above 17°C to form disordered micelles, which includes a range of states from (i) asymmetric swelling to (ii) micelle formation with only short-range liquidlike order. Above 30°C, network domains are formed as a result of macrophase separation due to dehydration of PNIPA blocks. As the temperature increased up to 40°C, the network domain is collapsed along a direction parallel to PNIPA-block-PEG interface, leading to increase in interfacial thickness and to macroscopic syneresis.